This invention relates to helium cooled superconducting magnet assemblies suitable for magnetic resonance imaging (hereinafter called "MRI"), and more particularly to an improved and simplified means within the helium vessel for recondensing the resultant helium gas back into liquid helium.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other liquid cryogen. The extreme cold insures that the magnet coils are maintained in superconducting operation, such that when a power source is initially connected to the magnet coils (for a period, for example, of 10 minutes) to introduce a current flow through the coils, the current will continue to flow through the coils even after power is removed due to the absence of electrical resistance in the coils, thereby maintaining a strong magnetic field. Superconducting magnet assemblies find wide application in the field of MRI.
Considerable research and development efforts have been directed at minimizing the need to replenish the boiling cryogen such as liquid helium. While the use of liquid helium to provide cryogenic temperatures is widely practiced and is satisfactory for MRI operation the provision of a steady supply of liquid helium to MRI installations all over the world has proved to be difficult and costly.
As a result, it is highly desirable to efficiently minimize the amount of liquid helium required and to minimize or eliminate the need to continually replenish the liquid helium supply as the liquid helium boils in providing the superconducting temperature for the superconducting magnet assembly. It is common practice to vent the helium gas resulting from the boiling helium, and to periodically replenish the helium supply.
Another problem encountered in the use of liquid helium to provide cryogenic temperatures for superconducting magnet assemblies is in storing the necessary reserve supply of liquid helium at cryogenic temperatures of 4.degree. K. (or close to absolute zero), and the related problem of periodically transferring a portion of the liquid helium in the storage reservoir to the liquid helium supply in the superconducting magnet.
Various attempts have been made to recycle the helium off-gas resulting from the boiling helium. They have not been entirely satisfactory. One difficulty encountered in the efforts to recondense helium gas has been that of access to, and servicing of, the recondensing equipment. It is extremely important that the MRI be maintained in operating condition if at all possible during any servicing of the recondensing equipment since discontinuance of superconducting operation to service the equipment can result in considerable down time and boiloff of the helium supply in the superconducting magnet assembly due to ramping down and the subsequent ramping up and shimming of the superconducting magnet to homogeneous superconducting operation. Such procedures are necessarily costly in time and material, not the least of which is the expense associated with the inability to utilize the expensive MRI equipment during the down time period.
One means of helium recondensing is disclosed in my copending patent application, entitled "Helium Recondensing Superconducting Magnet" filed Dec. 29, 1994, application Ser. No. 08/366,187 and assigned to the same assignee as the present invention.
The lack of thermal isolation between the cryocooler, the evacuated outer vessel, and the cold helium gas in the helium vessel has resulted in increased service time, higher cryogen consumption during service, and possible contamination of the recondensing contact surfaces and/or the cryocooler system. Accordingly, it is important to be able to avoid contamination and to provide recondensing equipment in which the cryocooler can be removed and serviced while maintaining superconducting operation of the superconducting magnet assembly and without upsetting the integrity of the vacuum in the outer vessel and the inner helium vessel.